ALTER ... ADD can increase the number of SSTables being read

With the following SSTables:

CREATE TABLE my_table (pk int, ck int, v1 int, PRIMARY KEY(pk, ck))

INSERT INTO my_table (pk, ck, v1) VALUES (1, 1, 1) USING TIMESTAMP 1000;
--> flush()
INSERT INTO my_table (pk, ck, v1) VALUES (1, 1, 2) USING TIMESTAMP 2000;
--> flush()
INSERT INTO my_table  (pk, ck, v1) VALUES (1, 1, 3) USING TIMESTAMP 3000
--> flush()

the following query:

SELECT pk, ck, v1 FROM my_table WHERE pk = 1 AND ck = 1

will only read the third SSTable.

If we add a column to the table (e.g. ALTER TABLE my_table ADD v2 int) and rerun the query, the query will read the 3 SSTables.

The reason for this behavior is due to the fact that C* is trying to read all the fetched columns to ensure that it will return a row if at least one of its column is non null.

In practice for CQL tables, C* does not need to fetch all columns if the row contains a primary key liveness as it is enough to guaranty that the row exists. By consequence, even after the addition of the new column C* should read only the third SSTable.